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                <ol class="chapter"><li class="chapter-item expanded affix "><a href="../../PREFACE.html">前言</a></li><li class="chapter-item expanded affix "><li class="part-title">快速上手</li><li class="chapter-item expanded "><a href="../../zh/tutorials/index.html"><strong aria-hidden="true">1.</strong> 快速上手</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../zh/tutorials/typescript-in-5-minutes.html"><strong aria-hidden="true">1.1.</strong> 5 分钟了解 TypeScript</a></li><li class="chapter-item expanded "><a href="../../zh/tutorials/asp.net-core.html"><strong aria-hidden="true">1.2.</strong> ASP.NET Core</a></li><li class="chapter-item expanded "><a href="../../zh/tutorials/asp.net-4.html"><strong aria-hidden="true">1.3.</strong> ASP.NET 4</a></li><li class="chapter-item expanded "><a href="../../zh/tutorials/gulp.html"><strong aria-hidden="true">1.4.</strong> Gulp</a></li><li class="chapter-item expanded "><a href="../../zh/tutorials/knockout.html"><strong aria-hidden="true">1.5.</strong> Knockout.js</a></li><li class="chapter-item expanded "><a href="../../zh/tutorials/react-and-webpack.html"><strong aria-hidden="true">1.6.</strong> React 与 webpack</a></li><li class="chapter-item expanded "><a href="../../zh/tutorials/react.html"><strong aria-hidden="true">1.7.</strong> React</a></li><li class="chapter-item expanded "><a href="../../zh/tutorials/angular-2.html"><strong aria-hidden="true">1.8.</strong> Angular 2</a></li><li class="chapter-item expanded "><a href="../../zh/tutorials/migrating-from-javascript.html"><strong aria-hidden="true">1.9.</strong> 从 JavaScript 迁移到 TypeScript</a></li></ol></li><li class="chapter-item expanded "><li class="part-title">手册</li><li class="chapter-item expanded "><a href="../../zh/handbook/index.html"><strong aria-hidden="true">2.</strong> 手册</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../zh/handbook/basic-types.html"><strong aria-hidden="true">2.1.</strong> 基础类型</a></li><li class="chapter-item expanded "><a href="../../zh/handbook/interfaces.html"><strong aria-hidden="true">2.2.</strong> 接口</a></li><li class="chapter-item expanded "><a href="../../zh/handbook/functions.html"><strong aria-hidden="true">2.3.</strong> 函数</a></li><li class="chapter-item expanded "><a href="../../zh/handbook/literal-types.html"><strong aria-hidden="true">2.4.</strong> 字面量类型</a></li><li class="chapter-item expanded "><a href="../../zh/handbook/unions-and-intersections.html"><strong aria-hidden="true">2.5.</strong> 联合类型和交叉类型</a></li><li class="chapter-item expanded "><a href="../../zh/handbook/classes.html"><strong aria-hidden="true">2.6.</strong> 类</a></li><li class="chapter-item expanded "><a href="../../zh/handbook/enums.html"><strong aria-hidden="true">2.7.</strong> 枚举</a></li><li class="chapter-item expanded "><a href="../../zh/handbook/generics.html"><strong aria-hidden="true">2.8.</strong> 泛型</a></li></ol></li><li class="chapter-item expanded "><li class="part-title">手册（进阶）</li><li class="chapter-item expanded "><a href="../../zh/reference/index.html"><strong aria-hidden="true">3.</strong> 手册（进阶）</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../zh/reference/advanced-types.html"><strong aria-hidden="true">3.1.</strong> 高级类型</a></li><li class="chapter-item expanded "><a href="../../zh/reference/utility-types.html"><strong aria-hidden="true">3.2.</strong> 实用工具类型</a></li><li class="chapter-item expanded "><a href="../../zh/reference/decorators.html"><strong aria-hidden="true">3.3.</strong> Decorators</a></li><li class="chapter-item expanded "><a href="../../zh/reference/declaration-merging.html"><strong aria-hidden="true">3.4.</strong> 声明合并</a></li><li class="chapter-item expanded "><a href="../../zh/reference/iterators-and-generators.html"><strong aria-hidden="true">3.5.</strong> Iterators 和 Generators</a></li><li class="chapter-item expanded "><a href="../../zh/reference/jsx.html"><strong aria-hidden="true">3.6.</strong> JSX</a></li><li class="chapter-item expanded "><a href="../../zh/reference/mixins.html"><strong aria-hidden="true">3.7.</strong> 混入</a></li><li class="chapter-item expanded "><a href="../../zh/reference/modules.html"><strong aria-hidden="true">3.8.</strong> 模块</a></li><li class="chapter-item expanded "><a href="../../zh/reference/module-resolution.html"><strong aria-hidden="true">3.9.</strong> 模块解析</a></li><li class="chapter-item expanded "><a href="../../zh/reference/namespaces.html"><strong aria-hidden="true">3.10.</strong> 命名空间</a></li><li class="chapter-item expanded "><a href="../../zh/reference/namespaces-and-modules.html"><strong aria-hidden="true">3.11.</strong> 命名空间和模块</a></li><li class="chapter-item expanded "><a href="../../zh/reference/symbols.html"><strong aria-hidden="true">3.12.</strong> Symbols</a></li><li class="chapter-item expanded "><a href="../../zh/reference/triple-slash-directives.html"><strong aria-hidden="true">3.13.</strong> 三斜线指令</a></li><li class="chapter-item expanded "><a href="../../zh/reference/type-compatibility.html"><strong aria-hidden="true">3.14.</strong> 类型兼容性</a></li><li class="chapter-item expanded "><a href="../../zh/reference/type-inference.html"><strong aria-hidden="true">3.15.</strong> 类型推论</a></li><li class="chapter-item expanded "><a href="../../zh/reference/variable-declarations.html"><strong aria-hidden="true">3.16.</strong> 变量声明</a></li></ol></li><li class="chapter-item expanded "><li class="part-title">手册（v2）</li><li class="chapter-item expanded "><a href="../../zh/handbook-v2/index.html"><strong aria-hidden="true">4.</strong> 手册（v2）</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../zh/handbook-v2/type-manipulation/template-literal-types.html"><strong aria-hidden="true">4.1.</strong> 模版字面量类型</a></li></ol></li><li class="chapter-item expanded "><li class="part-title">TypeScript 声明文件（.d.ts）</li><li class="chapter-item expanded "><a href="../../zh/declaration-files/index.html"><strong aria-hidden="true">5.</strong> 如何书写声明文件</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../zh/declaration-files/introduction.html"><strong aria-hidden="true">5.1.</strong> 介绍</a></li><li class="chapter-item expanded "><a href="../../zh/declaration-files/by-example.html"><strong aria-hidden="true">5.2.</strong> 举例</a></li><li class="chapter-item expanded "><a href="../../zh/declaration-files/library-structures.html"><strong aria-hidden="true">5.3.</strong> 库结构</a></li><li class="chapter-item expanded "><a href="../../zh/declaration-files/templates.html"><strong aria-hidden="true">5.4.</strong> 模板</a></li><li class="chapter-item expanded "><a href="../../zh/declaration-files/do-s-and-don-ts.html"><strong aria-hidden="true">5.5.</strong> 最佳实践</a></li><li class="chapter-item expanded "><a href="../../zh/declaration-files/deep-dive.html"><strong aria-hidden="true">5.6.</strong> 深入</a></li><li class="chapter-item expanded "><a href="../../zh/declaration-files/publishing.html"><strong aria-hidden="true">5.7.</strong> 发布</a></li><li class="chapter-item expanded "><a href="../../zh/declaration-files/consumption.html"><strong aria-hidden="true">5.8.</strong> 使用</a></li></ol></li><li class="chapter-item expanded "><li class="part-title">TypeScript for JavaScript</li><li class="chapter-item expanded "><a href="../../zh/javascript/type-checking-javascript-files.html"><strong aria-hidden="true">6.</strong> JavaScript 文件里的类型检查</a></li><li class="chapter-item expanded affix "><li class="part-title">工程配置</li><li class="chapter-item expanded "><a href="../../zh/project-config/index.html"><strong aria-hidden="true">7.</strong> 工程配置</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../zh/project-config/tsconfig.json.html"><strong aria-hidden="true">7.1.</strong> tsconfig.json</a></li><li class="chapter-item expanded "><a href="../../zh/project-config/project-references.html"><strong aria-hidden="true">7.2.</strong> 工程引用</a></li><li class="chapter-item expanded "><a href="../../zh/project-config/typings-for-npm-packages.html"><strong aria-hidden="true">7.3.</strong> NPM 包的类型</a></li><li class="chapter-item expanded "><a href="../../zh/project-config/compiler-options.html"><strong aria-hidden="true">7.4.</strong> 编译选项</a></li><li class="chapter-item expanded "><a href="../../zh/project-config/configuring-watch.html"><strong aria-hidden="true">7.5.</strong> 配置 Watch</a></li><li class="chapter-item expanded "><a href="../../zh/project-config/compiler-options-in-msbuild.html"><strong aria-hidden="true">7.6.</strong> 在 MSBuild 里使用编译选项</a></li><li class="chapter-item expanded "><a href="../../zh/project-config/integrating-with-build-tools.html"><strong aria-hidden="true">7.7.</strong> 与其它构建工具整合</a></li><li class="chapter-item expanded "><a href="../../zh/project-config/nightly-builds.html"><strong aria-hidden="true">7.8.</strong> 使用 TypeScript 的每日构建版本</a></li></ol></li><li class="chapter-item expanded "><li class="part-title">版本发布说明（Release Notes）</li><li class="chapter-item expanded "><a href="../../zh/release-notes/index.html"><strong aria-hidden="true">8.</strong> 新增功能</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-5.1.html"><strong aria-hidden="true">8.1.</strong> TypeScript 5.1</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-5.0.html"><strong aria-hidden="true">8.2.</strong> TypeScript 5.0</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-4.9.html"><strong aria-hidden="true">8.3.</strong> TypeScript 4.9</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-4.8.html"><strong aria-hidden="true">8.4.</strong> TypeScript 4.8</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-4.7.html"><strong aria-hidden="true">8.5.</strong> TypeScript 4.7</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-4.6.html"><strong aria-hidden="true">8.6.</strong> TypeScript 4.6</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-4.5.html"><strong aria-hidden="true">8.7.</strong> TypeScript 4.5</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-4.4.html"><strong aria-hidden="true">8.8.</strong> TypeScript 4.4</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-4.3.html"><strong aria-hidden="true">8.9.</strong> TypeScript 4.3</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-4.2.html"><strong aria-hidden="true">8.10.</strong> TypeScript 4.2</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-4.1.html"><strong aria-hidden="true">8.11.</strong> TypeScript 4.1</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-4.0.html"><strong aria-hidden="true">8.12.</strong> TypeScript 4.0</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-3.9.html"><strong aria-hidden="true">8.13.</strong> TypeScript 3.9</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-3.8.html"><strong aria-hidden="true">8.14.</strong> TypeScript 3.8</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-3.7.html"><strong aria-hidden="true">8.15.</strong> TypeScript 3.7</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-3.6.html"><strong aria-hidden="true">8.16.</strong> TypeScript 3.6</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-3.5.html" class="active"><strong aria-hidden="true">8.17.</strong> TypeScript 3.5</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-3.4.html"><strong aria-hidden="true">8.18.</strong> TypeScript 3.4</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-3.3.html"><strong aria-hidden="true">8.19.</strong> TypeScript 3.3</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-3.2.html"><strong aria-hidden="true">8.20.</strong> TypeScript 3.2</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-3.1.html"><strong aria-hidden="true">8.21.</strong> TypeScript 3.1</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-3.0.html"><strong aria-hidden="true">8.22.</strong> TypeScript 3.0</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-2.9.html"><strong aria-hidden="true">8.23.</strong> TypeScript 2.9</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-2.8.html"><strong aria-hidden="true">8.24.</strong> TypeScript 2.8</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-2.7.html"><strong aria-hidden="true">8.25.</strong> TypeScript 2.7</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-2.6.html"><strong aria-hidden="true">8.26.</strong> TypeScript 2.6</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-2.5.html"><strong aria-hidden="true">8.27.</strong> TypeScript 2.5</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-2.4.html"><strong aria-hidden="true">8.28.</strong> TypeScript 2.4</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-2.3.html"><strong aria-hidden="true">8.29.</strong> TypeScript 2.3</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-2.2.html"><strong aria-hidden="true">8.30.</strong> TypeScript 2.2</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-2.1.html"><strong aria-hidden="true">8.31.</strong> TypeScript 2.1</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-2.0.html"><strong aria-hidden="true">8.32.</strong> TypeScript 2.0</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-1.8.html"><strong aria-hidden="true">8.33.</strong> TypeScript 1.8</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-1.7.html"><strong aria-hidden="true">8.34.</strong> TypeScript 1.7</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-1.6.html"><strong aria-hidden="true">8.35.</strong> TypeScript 1.6</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-1.5.html"><strong aria-hidden="true">8.36.</strong> TypeScript 1.5</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-1.4.html"><strong aria-hidden="true">8.37.</strong> TypeScript 1.4</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-1.3.html"><strong aria-hidden="true">8.38.</strong> TypeScript 1.3</a></li><li class="chapter-item expanded "><a href="../../zh/release-notes/typescript-1.1.html"><strong aria-hidden="true">8.39.</strong> TypeScript 1.1</a></li></ol></li><li class="chapter-item expanded "><li class="part-title">破坏性改动（Breaking Changes）</li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/index.html"><strong aria-hidden="true">9.</strong> Breaking Changes</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-3.6.html"><strong aria-hidden="true">9.1.</strong> TypeScript 3.6</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-3.5.html"><strong aria-hidden="true">9.2.</strong> TypeScript 3.5</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-3.4.html"><strong aria-hidden="true">9.3.</strong> TypeScript 3.4</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-3.2.html"><strong aria-hidden="true">9.4.</strong> TypeScript 3.2</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-3.1.html"><strong aria-hidden="true">9.5.</strong> TypeScript 3.1</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-3.0.html"><strong aria-hidden="true">9.6.</strong> TypeScript 3.0</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-2.9.html"><strong aria-hidden="true">9.7.</strong> TypeScript 2.9</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-2.8.html"><strong aria-hidden="true">9.8.</strong> TypeScript 2.8</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-2.7.html"><strong aria-hidden="true">9.9.</strong> TypeScript 2.7</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-2.6.html"><strong aria-hidden="true">9.10.</strong> TypeScript 2.6</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-2.4.html"><strong aria-hidden="true">9.11.</strong> TypeScript 2.4</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-2.3.html"><strong aria-hidden="true">9.12.</strong> TypeScript 2.3</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-2.2.html"><strong aria-hidden="true">9.13.</strong> TypeScript 2.2</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-2.1.html"><strong aria-hidden="true">9.14.</strong> TypeScript 2.1</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-2.0.html"><strong aria-hidden="true">9.15.</strong> TypeScript 2.0</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-1.8.html"><strong aria-hidden="true">9.16.</strong> TypeScript 1.8</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-1.7.html"><strong aria-hidden="true">9.17.</strong> TypeScript 1.7</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-1.6.html"><strong aria-hidden="true">9.18.</strong> TypeScript 1.6</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-1.5.html"><strong aria-hidden="true">9.19.</strong> TypeScript 1.5</a></li><li class="chapter-item expanded "><a href="../../zh/breaking-changes/typescript-1.4.html"><strong aria-hidden="true">9.20.</strong> TypeScript 1.4</a></li></ol></li></ol>
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                        <h1 id="typescript-35"><a class="header" href="#typescript-35">TypeScript 3.5</a></h1>
<h2 id="改进速度"><a class="header" href="#改进速度">改进速度</a></h2>
<p>TypeScript 3.5 为类型检查和增量构建采用了几个优化。</p>
<h3 id="类型检查速度提升"><a class="header" href="#类型检查速度提升">类型检查速度提升</a></h3>
<p>TypeScript 3.5 包含对 TypeScript 3.4 的某些优化，可以更高效地进行类型检查。 在代码补全列表等类型检查驱动的操作上，这些改进效果显著。</p>
<h3 id="改进---incremental"><a class="header" href="#改进---incremental">改进 <code>--incremental</code></a></h3>
<p>TypeScript 3.5 通过缓存计算状态的信息（编译器设置、寻找文件的原因、文件在哪里被找到等等），改进了在 3.4 中的 <code>--incremental</code> 构建模式。<a href="https://github.com/Microsoft/TypeScript/pull/31101">我们发现重新构建花费的时间比 TypeScript 3.4 减少了 68%</a>!</p>
<p>有关更多信息，你可以查看这些 pull requests</p>
<ul>
<li><a href="https://github.com/Microsoft/TypeScript/pull/31100">缓存模块解析</a></li>
<li><a href="https://github.com/Microsoft/TypeScript/pull/31101">缓存 <code>tsconfig.json</code> 计算</a></li>
</ul>
<h2 id="omit-辅助类型"><a class="header" href="#omit-辅助类型"><code>Omit</code> 辅助类型</a></h2>
<p>TypeScript 3.5 添加了新的 <code>Omit</code> 辅助类型，这个类型用来创建从原始类型中移除了某些属性的新类型。</p>
<pre><code class="language-typescript">type Person = {
  name: string;
  age: number;
  location: string;
};

type QuantumPerson = Omit&lt;Person, &quot;location&quot;&gt;;

// 相当于
type QuantumPerson = {
  name: string;
  age: number;
};
</code></pre>
<p>使用 <code>Omit</code> 辅助，我们有能力复制 <code>Person</code> 中除了 <code>location</code> 之外的所有属性。</p>
<p>有关更多细节，<a href="https://github.com/Microsoft/TypeScript/pull/30552">在 GitHub 查看添加 <code>Omit</code> 的 pull request</a>, 以及<a href="https://github.com/microsoft/TypeScript/pull/31134">有关剩余对象使用 <code>Omit</code> 的更改</a>。</p>
<h3 id="改进了联合类型中多余属性的检查"><a class="header" href="#改进了联合类型中多余属性的检查">改进了联合类型中多余属性的检查</a></h3>
<p>在 TypeScript 3.4 及之前的版本中，会出现确实不应该存在的多余属性却被允许存在的情况。 例如，TypeScript 3.4 在对象字面量上允许不正确的 <code>name</code> 属性，甚至它的类型在 <code>Point</code> 和 <code>Label</code> 之中都不匹配。</p>
<pre><code class="language-typescript">type Point = {
  x: number;
  y: number;
};

type Label = {
  name: string;
};

const thing: Point | Label = {
  x: 0,
  y: 0,
  name: true // uh-oh!
};
</code></pre>
<p>以前，一个无区别的联合在它的成员上不会进行_任何_多余属性的检查，结果，类型错误的 <code>name</code> 属性溜了进来。</p>
<p>在 TypeScript 3.5 中，类型检查器至少会验证所有提供的属性属于_某个_联合类型的成员，且类型恰当，这意味着，上面的例子会正确的进行错误提示。</p>
<p>注意，只要属性类型有效，仍允许部分重叠。</p>
<pre><code class="language-typescript">const pl: Point | Label = {
  x: 0,
  y: 0,
  name: &quot;origin&quot; // okay
};
</code></pre>
<h2 id="--allowumdglobalaccess-标志"><a class="header" href="#--allowumdglobalaccess-标志"><code>--allowUmdGlobalAccess</code> 标志</a></h2>
<p>在 TypeScript 3.5 中，使用新的 <code>--allowUmdGlobalAccess</code> 标志，你现在可以从任何位置引用全局的 UMD 申明——甚至模块。</p>
<pre><code class="language-typescript">export as namespace foo;
</code></pre>
<p>此模式增加了混合和匹配第三方库的灵活性，其中库声明的全局变量总是可以被使用，甚至可以从模块内部使用。</p>
<p>有关更多细节，<a href="https://github.com/Microsoft/TypeScript/pull/30776/files">查看 GitHub 上的 pull request</a>。</p>
<h2 id="更智能的联合类型检查"><a class="header" href="#更智能的联合类型检查">更智能的联合类型检查</a></h2>
<p>在 TypeScript 3.4 以及之前的版本中，下面的例子会无效：</p>
<pre><code class="language-typescript">type S = { done: boolean, value: number }
type T =
  | { done: false, value: number }
  | { done: true, value: number };

declare let source: S;
declare let target: T;

target = source;
</code></pre>
<p>这是因为 <code>S</code> 无法被分配给 <code>{ done: false, value: number }</code> 或者 <code>{ done: true, value: number }</code>。 为啥？ 因为属性 <code>done</code> 在 <code>S</code> 不够具体——他是 <code>boolean</code>。而 <code>T</code> 的的每个成员有一个明确的为 <code>true</code> 或者 <code>false</code> 属性 <code>done</code>。</p>
<p>这就是我们单独检查每个成员的意义：TypeScript 不只是将每个属性合并在一起，看看是否可以赋予 <code>S</code> 。</p>
<p>如果这样做，一些糟糕的代码可能会像下面这样：</p>
<pre><code class="language-typescript">interface Foo {
  kind: &quot;foo&quot;;
  value: string;
}

interface Bar {
  kind: &quot;bar&quot;;
  value: number;
}

function doSomething(x: Foo | Bar) {
  if (x.kind === &quot;foo&quot;) {
    x.value.toLowerCase();
  }
}

// uh-oh - 幸运的是， TypeScript 在这里会提示错误!
doSomething({
  kind: &quot;foo&quot;,
  value: 123,
});
</code></pre>
<p>然而，对于原始的例子，这有点过于严格。 如果你弄清除 <code>S</code> 的任何可能值的精确类型，你实际上可以看到它与 <code>T</code> 中的类型完全匹配。</p>
<p>在 TypeScript 3.5 中，当分配具有辨别属性的类型时，如 <code>T</code>，实际上_将_进一步将类似 <code>S</code> 的类型分解为每个可能的成员类型的并集。 在这种情况下，由于 <code>boolean</code> 是 <code>true</code> 和 <code>false</code> 的联合，<code>S</code> 将被视为 <code>{done：false，value：number}</code> 和 <code>{done：true，value：number }</code>。</p>
<p>有关更多细节，你可以<a href="https://github.com/microsoft/TypeScript/pull/30779">在 GitHub 上查看原始的 pull request</a>。</p>
<h2 id="泛型构造函数的高阶类型推断"><a class="header" href="#泛型构造函数的高阶类型推断">泛型构造函数的高阶类型推断</a></h2>
<p>在 TypeScript 3.4 中，我们改进了对返回函数的泛型函数的推断：</p>
<pre><code class="language-typescript">function compose&lt;T, U, V&gt;(f: (x: T) =&gt; U, g: (y: U) =&gt; V): (x: T) =&gt; V {
  return x =&gt; g(f(x))
}
</code></pre>
<p>将其他泛型函数作为参数，如下所示：</p>
<pre><code class="language-typescript">function arrayify&lt;T&gt;(x: T): T[] {
  return [x];
}

type Box&lt;U&gt; = { value: U }
function boxify&lt;U&gt;(y: U): Box&lt;U&gt; {
  return { value: y };
}

let newFn = compose(arrayify, boxify);
</code></pre>
<p>TypeScript 3.4 的推断允许 <code>newFn</code> 是泛型的。它的新类型是 <code>&lt;T&gt;（x：T）=&gt; Box &lt;T []&gt;</code>。而不是旧版本推断的，相对无用的类型，如 <code>（x：{}）=&gt; Box &lt;{} []&gt;</code>。</p>
<p>TypeScript 3.5 在处理构造函数的时候推广了这种行为。</p>
<pre><code class="language-typescript">class Box&lt;T&gt; {
  kind: &quot;box&quot;;
  value: T;
  constructor(value: T) {
    this.value = value;
  }
}

class Bag&lt;U&gt; {
  kind: &quot;bag&quot;;
  value: U;
  constructor(value: U) {
    this.value = value;
  }
}

function composeCtor&lt;T, U, V&gt;(F: new (x: T) =&gt; U, G: new (y: U) =&gt; V): (x: T) =&gt; V {
  return x =&gt; new G(new F(x))
}

let f = composeCtor(Box, Bag); // 拥有类型 '&lt;T&gt;(x: T) =&gt; Bag&lt;Box&lt;T&gt;&gt;'
let a = f(1024); // 拥有类型 'Bag&lt;Box&lt;number&gt;&gt;'
</code></pre>
<p>除了上面的组合模式之外，这种对泛型构造函数的新推断意味着在某些 UI 库（如 React ）中对类组件进行操作的函数可以更正确地对泛型类组件进行操作。</p>
<pre><code class="language-typescript">type ComponentClass&lt;P&gt; = new (props: P) =&gt; Component&lt;P&gt;;
declare class Component&lt;P&gt; {
  props: P;
  constructor(props: P);
}

declare function myHoc&lt;P&gt;(C: ComponentClass&lt;P&gt;): ComponentClass&lt;P&gt;;

type NestedProps&lt;T&gt; = { foo: number, stuff: T };

declare class GenericComponent&lt;T&gt; extends Component&lt;NestedProps&lt;T&gt;&gt; { }

// 类型为 'new &lt;T&gt;(props: NestedProps&lt;T&gt;) =&gt; Component&lt;NestedProps&lt;T&gt;&gt;'
const GenericComponent2 = myHoc(GenericComponent);
</code></pre>
<p>想学习更多，<a href="https://github.com/microsoft/TypeScript/pull/31116">在 GitHub 上查看原始的 pull requet</a>。</p>
<h2 id="参考"><a class="header" href="#参考">参考</a></h2>
<ul>
<li><a href="https://github.com/microsoft/TypeScript-Handbook/blob/master/pages/release%20notes/TypeScript%203.5.md">原文</a></li>
</ul>

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